D. The "space race" was a Cold War competition by scientists.
D is the best option because it puts "space race" as the subject of the sentence. The subject is the main focus of the sentence. Option A does this as well, but the sentence is very wordy and does not do enough to draw attention to "the space race". Option B keeps the scientists as the subject of the sentence. Option C makes the "space race" part of a prepositional phrase which can be eliminated.